1. Field of the Invention
The present invention relates to a device for controlling fuel injection in feeding the fuel to an internal combustion engine depending upon the operating conditions by controlling the pressure of fuel inject ed by an injector and by controlling the driving time for opening the valve body. More particularly, the invention relates to a device for controlling fuel injection, which expands the dynamic range for controlling the amount of fuel by shortening the driving time when the valve body of the injector is opened or closed.
2. Prior Art
In the conventional direct cylinder injection-type device for controlling fuel injection in an internal combustion engine for vehicles, the valve body (electromagnetic valve) of the injector has been constituted in an internally opening type as taught in, for example, Japanese Patent Laid-Open No. 209807/1997.
In the device for controlling fuel injection of the direct cylinder injection type, a strict limitation is imposed on the timing for injecting the fuel compared with that of an ordinary intake pipe injection type and, besides, a strict limitation is imposed on the injection time for forming a favorable mixture, making it necessary to highly set the pressure of the fuel that is to be injected.
In setting the pressure of fuel, further, a dynamic range for controlling the amount of fuel must be maintained by taking into consideration the amount of fuel injection (maximum amount of injection) of when the vehicle carries a maximum load and a minimum amount of injection during the idling operation where a stratified charge combustion is taking place.
To maintain a dynamic range for controlling the amount of fuel, it can be contrived to variably set the fuel pressure.
It is considered that the method of variably setting the fuel pressure is effective in realizing an optimum state of injection in injecting the fuel through the injector.
When the injector having a valve of the internally opening type is used, the fuel pressure of the injected fuel acts in a direction to close the valve body. It therefore becomes necessary to so design the valve body of the injector that a desired valve-opening operation is accomplished at the time of a maximum fuel pressure.
In order to expand the linearity (dynamic range for controlling the amount of fuel) of the amount of injection for the injection pulse time (driving time for opening the valve body) of the injector, it is necessary to highly accurately control the opening and closing of the valve body by shortening the on/off operation time of the valve body of the injector.
To open (on) the valve body of the injector, therefore, a predetermined over-exciting current must be supplied at a sharp gradient to the electromagnetic coil for driving the valve body of the injector by using an injector drive means, and the over-exciting current must be maintained supplied over a period until the on-operation of the valve body is nearly completed.
After the on-operation of the valve body is completed, on the other hand, the electromagnetic coil produces a large electromagnetic attractive force. To maintain the valve body of the injector in a fully opened state, therefore, a minimum required holding current smaller than the over-exciting current needs be supplied to the electromagnetic coil.
In the conventional device taught in the above-mentioned publication, however, the over-exciting current and the holding current have been set to meet a maximum fuel pressure in a range of use. Therefore, the over-exciting current (and the holding current) at the time of the maximum fuel pressure are set even when the fuel pressure is controlled to lie on the lower side in the variable range.
Therefore, when, for example, the smallest fuel injection amount is to be stably controlled, it is necessary to set the fuel pressure on the lower side and to bring the time for exciting the electromagnetic coil as close to the valve-on time of the injector as possible. However, since the over-exciting period has been set to a maximum value (constant), the exciting time may become shorter than the over-exciting period.
In this case, there is substantially no period for feeding the holding current, and the state of being electromagnetically attracted by the over-exciting current shifts to the off state.
Accordingly, the valve body of the injector is opened for substantially an extended period of time, and an absolute value of the minimum amount of fuel that can be stably injected increases, resulting in a decrease in the dynamic range for controlling the amount of fuel.
This is because, in order to increase the dynamic range for controlling the amount of fuel, the valve-off time of the injector must be shortened as described above. When the valve is turned off from the state of supplying the over-exciting current (larger than the holding current), however, the initial valve-off motion is delayed compared to when the valve is turned off from the state of supplying minimum limit of current (holding current) necessary for attracting the valve body.
FIG. 4 is a diagram of waveforms illustrating the control operation of the injector by the conventional device for controlling fuel injection, and shows a relationship among the injection pulse J corresponding to the driving time for opening the valve body of the injector, an exciting current i supplied to the electromagnetic coil and an opening degree .theta. of the valve body in the injector.
Among the waveforms of FIG. 4, the waveform of a solid line represents a change in the time when the engine load is great (the amount of fuel injection is great and the fuel pressure is high) and the waveform of a broken line represents a change in the time when the engine load is small (amount of fuel injection is small and the fuel pressure is low).
In FIG. 4, the injection pulse J is formed by an ECU (electronic control unit) that is not shown depending upon the operating condition, is set to a relatively long pulse width t1 (see a solid line) when the engine load is large and is set to a relatively short pulse width t2 (see a broken line) when the engine load is small.
The exciting current i is set to an over-exciting current ie during an over-exciting period TE at the start of excitation of the electromagnetic coil, and is set to a holding current ih during the period of an injection pulse J after the over-exciting period TE has elapsed.
For convenience, the over-exciting current ie during the over-exciting period TE is represented by a flat waveform. In practice, however, the level of a peak current does not remain flat but varies since the current stems from the phenomenon of electric charge of a booster capacitor for sharp raising.
In this case, the over-exciting period TE is set to a predetermined value so as to become proper when the load is large (see the solid line) where the controlled fuel pressure is relatively high.
When the load is small (see the broken line), therefore, the over-exciting period TE becomes longer than the injection pulse width t2 as shown, and there is no period in which the exciting current i is decreased down to the holding current ih.
The opening degree .theta. of the valve body of the injector is quickly and fully opened (on state) in the initial stage of the operation for opening the valve due to the over-exciting current ie. Thereafter, the fully opened state is maintained when the load is large (see the solid line) due to the holding current ih, and the valve body returns to the fully closed (off state) due to the turn off of the exciting current i.
When it is the turn-off operation from the holding current ih as represented by the solid line, the opening degree .theta. of the valve body returns back to the fully closed state after a relatively short operation delay time .tau.1.
However, when it is the turn-off operation from the over-exciting current ie as represented by the broken line, the opening degree .theta. of the valve body returns to the fully closed state after the passage of an operation delay time .tau.2 which is longer than .tau.1.
When the load is small, the fuel pressure being controlled is lower than that of when the load is large. Therefore, the opening degree .theta. of the valve body driven by being supplied with the predetermined over-exciting current ie is brought to the fully opened state more quickly than when the load is large (see a solid line) as represented a broken line.
According to the conventional apparatus for controlling fuel injection as described above, the valve body of the injector is opened by supplying an over-exciting current to the electromagnetic coil for a predetermined over-exciting period irrespective of the driving time for opening the valve body of the injector or of the difference in the pressure for injecting the fuel. In an operating condition where the fuel is required in a small amount and the fuel pressure is low, therefore, the over-exciting period is set excessively to wastefully consume the electric power.
When the over-exciting period is set excessively, further, there does not substantially exist the period for supplying the holding current. Therefore, the valve-closing (off) operation of the injector is delayed, making it difficult to stably control the minimum amount of fuel injection and, hence, making it difficult to expand the dynamic range for controlling the amount of fuel.